1. Field of the Invention
A varistor produced using the method mentioned is used in medium- or high-voltage installations for measurement, protection or control purposes has a cylindrical resistance body which is arranged between two parallel electrodes and is made from a sintered ceramic or a polymer which has been highly filled with sintered ceramic granules with a varistor character.
The wintered ceramic or the sintered ceramic granules generally comprise(s) a zinc oxide which has been doped in a controlled manner with selected metals, such as Bi, Sb, Co and Mn.
The varistor is preferably used in surge arresters and has to be specified in such a way that it can conduct high-power current pulses which have been produced by lightening strikes or switching operations without being damaged. During the manufacturing process, such current pulses are applied to the electrodes of the varistor, in order to test their capacity to withstand high currents.
2. Discussion of Background
Methods producing such varistors are given in DE 34 05 834 C2 and EP 0,494,507 A1. In each of these methods, a cylindrical, ceramic resistance body based on zinc oxide is produced and an electrode is applied to each of the two parallel, planar end faces of the resistance body.
In the method described in DE 34 05 834 C2, circumferential steps are ground off the resistance body in the peripheral areas of both end faces. Then, the resistance body is provided with an insulating material which covers the circumferential face and the steps. After that, the end faces and some of the insulating material which has been applied to the steps are ground off. Finally, the metal electrodes are applied to the end faces in such a manner that they partly overlap the steps which have been filled with the insulating material but do not reach all the way to the edge of the end face. This method is extremely complex and, in addition, is susceptible to faults, since metal splashes may be formed in the peripheral area when the electrode material is applied, which splashes may lead to dielectric sparkovers when high-field current is applied. In addition, the incomplete coverage of the electrodes results in local overheating of the current density or the electric field in the resistance body, which overheating reduces the dielectric strength of a varistor which has been designed in this way.
In the method described in EP 0,494,507 A1, each of the electrodes is applied all the way to the edge of the end faces of the resistance body. Since, in a varistor of this type, each of the two electrodes extends over the entire end face of the resistance body, a homogenous electric field is formed inside the resistance body when a high current is conducted for a short time. This results in a uniform current density and therefore also in uniform heating of the varistor. Since the unprotected resistance body has sharp edges and points in the area of the outer boundaries of the end faces, and since the electrode material, which runs to as far as the outer boundaries, may pass into the circumferential surface of the resistance body, a ring made from a polymer with a high dielectric constant and with a high temperature stability is positioned on the circumferential surface of the resistance body. This ring ensures that the electric field is reduced in the circumferential surface, thus avoiding undesirable sparkovers. Again, such a method for producing varistors is extremely expensive and complex.